1. Field of the Invention
The present invention relates to the selective hydrogenation of dienes in mixed streams of olefin containing hydrocarbons, such as butadiene in a mixed C4 stream with minimum loss of monoolefins. More particularly the invention relates to hydrogenation of butadiene in multi-phase reactions where a hydraulic regime is utilized, which provides pulsations, to yield greater mixing and associated interfacial mass transfer and heat transfer. By obtaining a desired vapor and liquid mass flux in a downflow reactor process, fluid pulsations can be induced. Most particularly the invention relates to butadiene hydrogenation with minimum olefin loss in a downflow, boiling point, vapor induced pulse reactor.
2. Related Information
The pulse flow regime has been studied in regard to trickle-bed reactors. Generally, “trickle-bed reactor” refers to a solid particulate packed bed downflow reactor operating in the trickle flow or gas continuous regime. A hydraulic map called a Baker plot is often used to indicate the mass fluxes required to obtain a given hydraulic regime (i.e., trickle, pulse, bubble flow). Weekman, V. W., Jr., and J. E. Myers, “Fluid-Flow characteristics of concurrent gas-liquid flow in packed beds”, AlChE Journal, 10, 951 (1964) provides a map of the various hydraulic regimes found in packed beds. These pulses yield turbulent mixing within the reactor system and provide a higher level of mass and heat transfer, not typical of commercial reactors which tend to operate in the hydraulic region known as trickle flow.
Pulse flow in a mixed phase reactor is defined as a hydraulic region in which waves of liquid continuous slugs of material move down the reactor. In between each wave is a region of flow which is considered to be gas continuous. The pulses are discrete bands of material with higher overall density than that of the material both in front and behind the pulse or wave. By changing the overall liquid and vapor mass fluxes within this hydraulic region, the frequency at which the pulses flow down the reactor can be manipulated. Higher overall mass flux yields higher frequency pulses, and lower mass flux yields lower frequency pulses. The mechanism for development of this type of flow is not due to oscillations provided by some type of mechanical device, rather it is a known two-phase (vapor/liquid) hydraulic region which is a function of the relative vapor and liquid velocities.
Fukushima, S. and Kusaka, K., J. of Chem. Eng. Japan 10, p. 468 (1977) provided Equations 1 and 2, which demonstrate the increase in mass transfer as one moves into the pulse flow regime. The difference between the liquid to gas mass transfer coefficient for trickle flow and pulsing flow can be seen from the following two equations where equation (1) is for trickle flow and equation (2) is for pulsing flow:kLai=2.05p0.2Rel0.73ReG0.2Sc0.5(dp/D)0.2(1-hext)Dml/dp2  (1)kLai=0.11RelReG0.4Sc0.5(dp/D)−0.3(1-hext)Dml/dp2  (2)where:
kL is the mass transfer coefficient
ai is the specific interfacial surface area
Sp is the external surface of particle divided by the square of the particle diameter
Rel is the modified Reynolds number of the liquid (density removed)
ReG is the modified Reynolds number of the gas (density removed)
Sc is Schmidt number (ratio of the momentum diffusivity to the mass diffusivity
dp is the particle diameter,
D is the diameter of the reactor,
hext is the liquid hold up (ratio of the volume of liquid held up in the reactor over the total reactor volume), and
Dml is the molecular diffusivity of the gas
This is presented graphically in FIG. 1 where the ratio of pulse flow mass transfer coefficient to trickle bed mass transfer coefficient is shown to increase with the Reynolds number of the liquid or gas. The Reynolds number of either the liquid or gas is directly proportional to the flow rate, all other variables (diameter of reactor, density and viscosity of component) being constant.
Schuster et al U.S. Pat. No. 4,288,640 identifies a narrow region within the Baker plot where heat transfer benefit occurs as one increases the mass fluxes of the gas and liquid and approaches pulse flow. This region of operation is called transitional flow. Transitional flow represents a narrow region of mass fluxes between trickle flow and pulse flow. This region is essentially on the transition line of the flow map separating pulse flow from trickle flow, which lies at a point where a small change in liquid flow causes a relatively large change in differential pressure drop across the bed. Schuster et al list a range of ΔP/L of twice the ΔP/L obtained during trickle bed operation and characterizes the pulse region as one where fluctuations in the pressure difference across the reactor occur and the pressure fluctuations as having the same frequency as the pulses. It is known, however, that the pulse regime extends far beyond the differential pressure drop change of twice trickle flow.
A plot of gas vs. liquid mass flux for pilot and commercial scale reactors was presented in “Trickle Bed Reactors”, Charles Satterfield, AlChE Journal, Vol. 21, No. 2, March 1975, pp. 209–228. The author observed that the operating region for the pilot scale reactors was in trickle regime; whereas, some commercial reactors operated in the pulse region. This suggests that during scale-up to commercial size some commercial reactors were inadvertently designed to operate in the pulse region, since at the time, running in pulse mode was considered to lead to undesirable hydraulic instability and breakup of catalyst particles in the packed bed.
With typical trickle bed reactors, like those used for hydrotreating using a solid catalyst, the main resistance toward the desired hydrogenation includes: 1) mass transfer from the gas phase into the liquid phase, 2) mass transfer from the liquid phase onto and off of the catalyst surface, 3) diffusion into and out of the catalyst pore space, 4) adsorption of the reactants onto the catalyst surface, 5) chemical reaction, and 6) desorption of the products into the pore space.
Although reactor operation in the pulse flow region may provide interesting mass transfer benefits, two main concerns exist. The first addresses fixed bed catalyst life. Due to the high liquid and vapor rates, vibration of the fixed bed may occur causing physical catalyst degradation and abrasion over time. Secondly a problem in scale up from pilot plant units may be encountered. The small size of pilot plant reactors induces wall effects which occlude space for radial pulse dispersion and it is not known whether larger diameter reactors provide an equivalent flow pattern at the same liquid and vapor velocities. It is an advantage of this invention that a multi-phase co-current flow reactor system that operates efficiently in the pulse flow region is provided.